Inorganic-organic polymer composite materials are used in a wide variety of applications including structural materials, high performance composites, optical components, aerospace, biomedical implants and dental applications. Generally, composites are employed where performance requirements are demanding and not easily fulfilled with traditional structural materials. For example, inorganic materials, such as glass, ceramic and stone, are very hard, scratch resistant and even sometimes transparent (e.g., glass), but suffer from the fact that they are very heavy and brittle. Polymers, conversely, are light and durable, but have poor hardness, abrasion and wear resistance. Composites, made from the combination of inorganic materials and polymers, may have properties that lie in between, providing materials that are simultaneously strong but lightweight, hard but flexible, abrasion resistant and durable.
In order to achieve such properties, in practice, hard inorganic materials are mixed into polymers, or polymer precursors, monomers and/or oligomers, referred to as resins, and the mixture is then cured to form a composite. The inorganic materials are often referred to as fillers, although they may play the salient role in determining the properties of the composite. Glass or ceramic fillers are commonly used because they are low cost, and, more recently, nanomaterial fillers have been used to provide composites with performance advantages. Hereafter, inorganic addenda are referred to as performance additives.
Performance additives are an extremely important component of coatings and composite formulations. They impart a wide variety of properties to the end products including strength and toughness, scratch and mar resistance, UV absorption, optical properties, anticorrosion, and biocompatibility (for medical based coatings). Typical performance addenda are comprised of inorganic metal oxides, such as silica, titania, alumina, and zinc oxide; they may be categorized according to their size: micron-sized (0.2-100 μm) or nano-sized (1-200 nm).
There are several problems or difficulties generally experienced in mixing performance additives into polymers. First, polymers or polymer precursors may be viscous and the addition of performance materials only increases the viscosity and limits the loading of material that may be achieved, and creates difficulty in handling, molding and crafting the composite into an article of commerce. Second, inorganic performance materials generally have a high surface energy compared to resins, and the mismatch in the interfacial energy may cause the inorganic materials to agglomerate and/or aggregate, making a homogeneous dispersion difficult or impossible to achieve. This problem is particularly acute if the particle size of the performance additive is small, especially in the case of nanomaterials, i.e., materials with a particle size between about 1 to 200 nm.
The polymer industry is transforming from composites that are polymerized, or cured, using heat (thermal set polymers) to those that are cured using ultraviolet or visible light, or low energy electrons (UVEB). UVEB curable resins offer tremendous energy and waste savings to the coatings and composites industries because they are polymerized (cured) directly with light and also because they generally do not contain volatile diluents, such as solvents or carriers that may be considered hazardous air pollutants. UVEB curing is far more energy efficient, since it overcomes the thermal loss that is prevalent in conventional thermoset coating systems. Ironically, the fundamental advantages of UVEB systems, where a solventless medium is cured rapidly by radiation, are also the source of significant system limitations.
Light curing requires that the coating and/or object must be sufficiently transparent in the spectral region of curing, since the penetration depth and absorption of the curing radiation is essential to achieve rapid and efficient curing. This limits the performance additives (fillers, stabilizers, functional additives, and coating aids) that can be added to UVEB systems, since the additives must also fulfill the requirement of being optically transparent in the curing region of the spectrum. While there are some types of addenda that meet this requirement, their formulation into UVEB resins can be very difficult, since these systems do not contain diluents or volatile components.
Diluents (solvents and volatiles) act as dispersion aids and carriers that enable integration of a wide variety of functional additives into paints and coatings formulations. Diluents give the formulator tools with which to adjust viscosity and rheology, disperse solids and overcome formulation incompatibilities. These factors, in combination with the absorption requirements of UVEB formulations, greatly limit the performance additives that can be utilized.
The dental industry, primarily due to health concerns, is rapidly transitioning dental restoratives (e.g., cavity fillings, dental restorations) from the conventional mercury-based amalgams to highly filled, light curable, polymer-based composites. Polymer-based composites are safer and better match the color and appearance of human tooth enamel, but are often softer, not as strong or as durable as the traditional metal amalgams. To resolve these problems, manufacturers have developed microfilled polymer composites that have strength, hardness and durability close to that of the conventional amalgams. To achieve the performance requirements, polymers are highly filled at loadings of 70-80% by weight performance additives. It is generally desirable that the filling percentage be as high as possible to approximate the hardness of teeth, however, loadings greater than about 80% are difficult if not impossible to achieve.
From the patient's perspective, the aesthetic quality of the restoration is extremely important, since teeth are an important part of personal appearance. Matching the aesthetic quality of natural human enamel is difficult, since teeth, although opaque, have a translucent or opalescent quality that provides luster and visual brilliance. To achieve these qualities, some dental restorative manufacturers have developed performance additives that are closely matched in refractive index to the polymers used to prepare dental restoratives. The more closely index-matched the performance additives are to the polymer, the greater the translucency and aesthetic quality of the restoration. Because the two materials share the same index of refraction, there is little scatter of light and the resulting restorative composite resembles natural teeth in optical translucency and appearance. This also has the added benefit that it increases light penetration and the curing depth of the composite.
There are two types of fillers that are used in dentistry to give high optical translucency and aesthetic quality. The first is a glass or melt derived filler that is produced by melting a glass composition of known refractive index, rapidly cooling or quenching the melt, (for example into cold water) into a glass, and then pulverizing the glass to a given particle size, usually between about 0.4 and 10.0 microns. This process produces amorphous, shard-like particles of low surface area, usually between about 1-10 m2/g. A prevalent example of this type of filler is barium glass.
The second is a microporous filler that is produced from the thermal treatment of mixtures of colloidal dispersions of oxides, such as silica, zirconia and alumina. The refractive index is controlled through control of the composition. This process was first developed by Mabie et al. U.S. Pat. Nos. 4,217,264 and 4,306,913, to produce amorphous, microporous mixed oxides of silica and zirconia, and later by Randklev U.S. Pat. No. 4,503,169 to produce crystalline, microporous mixed oxides of silica, zirconia, and other oxides.
The microporous fillers are highly fused materials consisting of silica and other oxide particles and, because they are processed at a temperature below the melting temperature of any of the components, they are porous and have a high surface area. As Randklev pointed out, the surface area may be as high as 200 m2/g and the average pore volume may be as high as 40% of the volume of the filler. These microporous fillers have received much attention because of their numerous advantages, including improved finish, gloss, strength, and abrasion resistance.
There is a problem, however, in that for microporous fillers, both the internal porosity and surface area is high, and it is difficult to achieve high loadings of the porous fillers in dental monomers. The internal pores soak up the organic resin, limiting the fraction of resin that may keep the suspension in a fluid state, and the viscosity rises exponentially making the paste unworkable.
There is an additional problem with modern dental composite restorations. Modern dental materials contain a liquid, polymerizable resin in the form of monomers, or monomer mixtures, as an essential component. It is known that, during polymerization, a volume contraction takes place. The volume contraction is often called shrinkage and is attributable to the development of covalent bonds between the monomer molecules during polymerization, whereby the distance between the molecules is decreased. During the preparation of pre-shaped parts, the polymerization shrinkage has a very disadvantageous effect on the dimensional stability and the mechanical properties of the molded bodies. In the case of adhesives and gluing compounds, the polymerization shrinkage adversely affects the adhesion properties and the bonding strength, which deteriorates the adhesion between restoration material and the natural tooth substance of dental materials. Voids and cracks may result which become reservoirs for bacteria and encourage the development of secondary caries.
In order to reduce the polymerization shrinkage of dental materials, the industry has developed pre-polymerized fillers in which a mixture of inorganic fillers and monomers is polymerized and then ground to the desired size and then mixed again with monomers to form a flowable mixture that can be molded in tooth restorations. Because a portion of the polymer is pre-polymerized, the amount of shrinkage is slightly reduced. The preparation and use of such fillers, sometimes called pre-polymers or composite fillers, has been described in the patent and scientific literature.
U.S. Pat. No. 5,356,951 to Yearn et al. discloses a composition for dental restorative material comprising: (a) a first methacrylate or acrylate monomer having at least one unsaturated double bond, (b) (i) a composite filler obtained by curing and pulverizing a mixture of a first glass powder component having a maximum particle diameter of 10 μm or less and a mean particle diameter of 0.1 to 5 μm with a second methacrylate or acrylate monomer having at least one unsaturated double bond, (ii) a second glass powder component having a maximum particle diameter of 10 μm or less and a mean particle diameter of 0.1 to 5 μm, and iii) a fine particle filler having a mean particle diameter of 0.01 to 0.04 μm, and a photo-polymerization initiator. The filler described is a non-porous filler.
U.S. Pat. No. 7,091,258 to Neubert et al. discloses a composition comprising: (i) 10 to 80 wt. % organic binder; (ii) 0.01 to 5 wt. % polymerization initiator; (iii) 20 to 90 wt. % particulate composite filler, comprising a polymerized mixture of organic binder and inorganic filler, the composite filler particles having an average particle size of 20 to 50 μm, each wt. % of (i), (ii), and (iii) relative to the total mass of the composition; and wherein the composition contains at most 10 wt. % composite filler particles having a size of <10 μm, relative to the total mass of the particulate composite filler in the composition. There is a problem, however, in that the material of Nuebert et al. requires extensive grinding in order to be used as a dental filler, and, at best, a relatively large particle size (20-50 μm) is achieved.
EP 0 983 762 A1 to Katsu discloses an organic-inorganic composite filler for use in dentistry. The composite filler is prepared by curing a mixture of a particulate filler with an average particle size of 20 nm or less and a methacrylate or acrylate monomer with a viscosity of 60 cP or more and pulverizing the cured mixture. The materials are said to be characterized by good polishability and good mechanical properties and have a smoothness and transparency corresponding to the natural tooth.
U.S. patent application Ser. No. 13/583,687 to Yamazaki et al. discloses an organic/inorganic composite filler that contains: inorganic agglomerated particles comprising agglomerations of inorganic primary particles having a mean diameter between 10 and 1000 nm; an organic resin phase that covers the surface of each inorganic primary particle and binds the inorganic primary particles to each other; and intra-agglomerate voids, formed between the organic resin phase covering the surface of each inorganic primary particle, with a pore volume (here, pore refers to holes with diameters between 1 and 500 nm) between 0.01 and 0.30 cm3/g as measured by mercury intrusion porosimetry. There is a problem, however, in that Yamazaki et al. is directed toward bonding or gluing together discreet primary particles with a polymer phase and does not provide high transparency filler materials.